The monitoring of moisture in soils by measuring conductivity between two electrodes is well known. An example is found in Larson U.S. Pat. No. 4,531,087 issued Jul. 23, 1985. The Larson device is typical of sensors which have two spaced-apart electrodes packed in a porous medium, protected by a filter medium which excludes particulates but passes moisture. The conductivity of the medium between the electrodes is measured to reflect the level of moisture in the porous medium.
The conductivity (or electrical resistance) of the region occupied by the porous medium is not a strictly linear function of the moisture content in the soil. Still, properly interpreted it can give the agronomist or gardener valuable information regarding the moisture at some depth in the soil, and can give him guidance concerning the need for irrigation or watering. Simplistically stated, when the porous region is completely dry, there will be no, or at the most minimal, conductivity. A very low or no meter reading will result if current flow is measured. If resistance to a given applied voltage is measured, the inverse of this reading will occur. When the sensor is fully wetted, the conductivity will be at a maximum, and there will be a higher meter reading reflecting this fact.
There is a serious problem. A sensor to be economically practical must be able to function in a wide variety of soils and be responsive to a wide variety of waters. No matter how wet the sensor would be with distilled water, for example, there would always be a low or no reading because conductivity of such water is very low.
It is an object of this invention to provide a sensor whose conductivity is less affected by the variations in concentration of the salts dissolved in soil waters before it reaches the sensitive part of the sensor, and which assures that the water will always have at least some conductivity. The sensor's output is therefore more reliably related to the moisture content of the soil than in previously known sensors such as the Larson sensor.
It is tempting to assume that a sensor of this type should have only two readings- one responsive to no moisture, and another responsive to the presence of sufficient moisture to form a circuit between the two electrodes. Fortunately the actual pattern Is somewhat more complicated. While a graph relating conductivity to moisture content is likelier to be steep in any event, it is not an abrupt curve. As the soil moisture decreases, the conductivity will begin to decrease. It is this decrease and the rate of decrease which the agronomist will watch for, because it will alert him to the fact that the soil is drying out as the consequence of suction exerted by the crop. Properly instrumented, this sensor will give the user adequate notice of the change of moisture content and thus of the demands of the crop. Also, when water is thereafter applied to the soil, a gradual increase in conductivity will signal the rise in the moisture content at the sensor. The rate of change gives information valuable in determining the amount of water to be applied, and at what rate, to satisfy the needs of the crop, as well as to avoid over saturation of the ground with attendant waste of water or excessive loss of water, which could lead to crop damage.
Thus, this sensor is intended as a measurement of the soil moisture at the sensor s level, and of its rate of change.
This invention enables a single sensor to be used for the above purposes in a wide variety of waters, while still providing readings which are consistent with a given meter calibration that can be established at the factory. This is accomplished by providing in the sensor itself a substantial source of electrolyte ions which will assure conductivity whenever moisture is present. It is provided in such a way as to remain effective over a long period of time.
There are other problems solved by this invention. These sensors are placed in soils Which are subjected to occasional variable physical loads, and to stray electrical currents. One can damage or destroy a sensor, and the other can cause faulty or stray readings. This invention provides a strong conductive metal case which resists the physical loads, and which provides a shield that excludes many atray currents. As a consequence, this invention provides a remarkably improved and long-lived sensor to detect soil moisture, and to detect the change and rate of change in moisture content at critical times.
It is another object of this invention to provide a sensor structure which includes only a minimum number of parts, and which can be manufactured to a high degree of consistency from one sensor to another.